Introduction to Earthquakes
Professor Jeffery Seitz
Department of Earth & Environmental Sciences
California State University East Bay
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Fault Geometry
Earthquakes
Fault Creep
Seismology
Earthquakes and Plate Tectonics
Intensity and Magnitude
Earthquake Hazards
1
Fault Geometry
A fault is a fracture in the crust on which there has been
appreciable displacement.
Hanging wall — rock surface immediately above the fault surface.
Footwall — rock surface
immediately below the
fault surface.
There are several types of
faults depending upon the
geometry of the fault:
1. Dip-slip faults
•normal
•reverse
•thrust
2. Strike-slip faults
3. Oblique slip faults
Detailed view of a normal fault. White strip
running from upper left to lower right corner
of picture is material dragged along fault
plane.
Strike and Dip
Geologists have developed a
convenient way to portray the
orientation of planar surfaces
such as faults in the Earth on
maps.
Strike is the compass direction
of the line produced by the
intersection of an inclined fault
with a horizontal plane.
Dip is the angle of inclination of
the surface of the fault
measured from a horizontal
plane
Dip Slip Faults
Dip slip faults are faults where the movement is
parallel to the dip of the fault surface or plane.
There are two main types of dip slip faults:
•normal faults
•reverse faults
In normal faults,
the hanging
wall block
moves down
relative to the
footwall block.
IRIS
USGS
IRIS
Normal faults accommodate or are caused by extension of the
crust — the Basin and Range province (E. California and
Nevada) is caused by crustal extension and normal faults.
Dip Slip Faults
Dip slip faults are faults where the movement is
parallel to the dip of the fault surface or plane.
There are two main types of dip slip faults:
•normal faults
•reverse faults
In normal faults,
the hanging
wall block
moves down
relative to the
footwall block.
IRIS
USGS
IRIS
Normal faults accommodate or are caused by extension of the
crust — the Basin and Range province (E. California and
Nevada) is caused by crustal extension and normal faults.
Dip Slip Faults
In reverse faults, the hanging wall block moves up
relative to the footwall block.
Reverse faults accommodate shortening
(compression) of the crust — compressional forces
that form these faults generally produce folds in
association with them.
IRIS
USGS
IRIS
Dip Slip Faults
In reverse faults, the hanging wall block moves up
relative to the footwall block.
Reverse faults accommodate shortening
(compression) of the crust — compressional forces
that form these faults generally produce folds in
association with them.
IRIS
USGS
IRIS
Dip Slip Faults
Thrust faults are reverse faults where the angle
of dip is <45°.
USGS
IRIS
Dip Slip Faults
Thrust faults are reverse faults where the angle
of dip is <45°.
USGS
IRIS
Strike-Slip Faults
In strike slip faults, the displacement
(movement) is horizontal and parallel
to the strike of the fault trace.
IRIS
USGS
USGS
Right-lateral strike-slip
faults have a sense of
movement that the
crustal block on the
opposite of the fault
has moved to the right
as you face the fault
(ex. San Andreas fault
system).
Strike-Slip Faults
In strike slip faults, the displacement
(movement) is horizontal and parallel
to the strike of the fault trace.
IRIS
USGS
USGS
Right-lateral strike-slip
faults have a sense of
movement that the
crustal block on the
opposite of the fault
has moved to the right
as you face the fault
(ex. San Andreas fault
system).
Strike-Slip Faults
In left-lateral strike-slip faults, the
movement is such that the block on the
opposite side of the fault appears to
have moved to the left as you face the
fault .
IRIS
USGS
USGS
The Great Glen
fault (Scotland) is
an example of a
left lateral fault.
Strike-Slip Faults
In left-lateral strike-slip faults, the
movement is such that the block on the
opposite side of the fault appears to
have moved to the left as you face the
fault .
IRIS
USGS
USGS
The Great Glen
fault (Scotland) is
an example of a
left lateral fault.
Strike-Slip Faults
A transform fault is a
type of strike-slip fault
where the motion is
between two large
crustal (tectonic) plates
(ex. San Andreas fault
system).
Large strike-slip fault
systems commonly
consist of roughly
parallel branches that
define a fault zone up to
several kilometers wide.
W. P. Irwin, USGS
It is easy to see offset
features when there has been
strike-slip displacement at the
Earth's surface.
In this photograph, the fence
line near Point Reyes, CA was
displaced approximately 8.5
feet by the 1906 San
Francisco earthquake.
The dashed line indicates the
fault trace and the sense of
motion is right-lateral.
USGS
Other offset features such
as stream channels
indicate strike-slip motion.
In this photograph, the
stream channel on the
Carrizo Plain (central
California) has been
displaced (right-lateral).
Displaced streams are
common in the East Bay
Hills adjacent to the
Hayward fault such as at
the Hayward Memorial
Park.
USGS
During strike-slip motion along the
San Andreas fault, rocks are
crushed and are more easily
eroded. This commonly results in
linear valleys or troughs that mark
the location of the fault.
This photograph shows a linear
valley (or rift) caused by the San
Andreas in the Mecca Hills in
southern California.
In the Bay Area, the linear valley
that the Warren freeway (Highway
13) runs through is a rift valley
caused by the Hayward fault.
Imagine the damage to the Warren
freeway if a surface rupture
occurred along the Hayward fault.
USGS
Oblique-Slip Faults
Strike-slip and dip-slip faults represent end-member models of
movement along faults. Some faults may exhibit both strike-slip
and dip-slip motion and are classified as oblique-slip faults.
Most faults actually
exhibit some degree of
oblique-slip motion.
USGS
IRIS
Oblique-Slip Faults
Strike-slip and dip-slip faults represent end-member models of
movement along faults. Some faults may exhibit both strike-slip
and dip-slip motion and are classified as oblique-slip faults.
Most faults actually
exhibit some degree of
oblique-slip motion.
USGS
IRIS
Bay Nature
Although the Hayward fault is classified as a strike-slip fault, a
small degree of dip-slip motion has created a scarp on the
western edge of the East Bay Hills where the block to the west
has moved down relative to the east.
Earthquakes
An earthquake is the vibration of
the Earth produced by an abrupt
release of energy.
Most commonly, an earthquake is
the result of slippage along a fault
in the Earth’s crust.
A fault is a fracture in the crust on
which there has been appreciable
displacement.
The energy is released in all
directions from its source known
as the focus.
The epicenter of an earthquake is
the point on the Earth’s surface
directly above the focus.
Earthquakes
An earthquake is the vibration of
the Earth produced by an abrupt
release of energy.
Most commonly, an earthquake is
the result of slippage along a fault
in the Earth’s crust.
A fault is a fracture in the crust on
which there has been appreciable
displacement.
The energy is released in all
directions from its source known
as the focus.
The epicenter of an earthquake is
the point on the Earth’s surface
directly above the focus.
Earthquakes
An earthquake is the vibration of
the Earth produced by an abrupt
release of energy.
Most commonly, an earthquake is
the result of slippage along a fault
in the Earth’s crust.
A fault is a fracture in the crust on
which there has been appreciable
displacement.
The energy is released in all
directions from its source known
as the focus.
The epicenter of an earthquake is
the point on the Earth’s surface
directly above the focus.
Elastic Rebound
As tectonic forces deform
rocks on both sides of a
fault, the rocks bend and
store elastic energy.
Eventually the frictional
resistance is overcome
(exceed shear strength of
rocks) and slippage occurs.
The built-up strain is
released and the deformed
rocks "snap back." An
earthquake results from
vibrations as the rock
snaps back into shape elastic rebound.
IRIS
In the slip board
activity, the
deformation of the
rocks is represented
by the stretching
(deformation) of the
rubber band. Rocks
will store elastic
energy like the
rubber band.
Eventually, the frictional resistance is overcome and slippage of the
sled occurs on the slip board. Likewise in rocks, the strength of the
rocks is exceeded and slippage occurs.
When the sled on the slip board moves, the built-up strain in the rubber
band is released and it begins to snap back to its original shape (more
relaxed). Likewise in rocks, the built-up strain is release and the
17
deformed rocks “snap back.”
Most earthquakes are
caused by the rapid release
of elastic energy.
These images show the
rupture surface (thrust fault)
from the Chi-chi earthquake
in Taiwan (1999).
Images from the Research Center for Earthquake Prediction
(Kyoto University)
Foreshocks and Aftershocks
Small earthquakes, known as foreshocks, may precede a major
earthquake by hours, days, or years. These foreshocks are actively
investigated as a means of predicting major earthquakes.
After a major earthquake, adjustments may be made along a fault
resulting in the generation of many smaller earthquakes known as
aftershocks.
Aftershocks are generally weaker
but may destroy buildings
damaged during the main shock.
After the Loma Prieta earthquake
(magnitude 7.1), a magnitude 5.2
aftershock occurred within 2.5
minutes. There were thousands of
aftershocks; 20 aftershocks with a
magnitude greater than 4.0
occurred within one week of the
main shock.
The movie shows foreshocks and aftershocks associated with the
Landers earthquake (M7.3, 1992) in S. California. The map shows
the area near the Landers earthquake epicenter. Some highways
(dark blue) and surface traces of faults (light bluegreen lines) are
shown. The animation shows earthquake epicenters as colored
spots, appearing and disappearing with time.
SCEC
The movie shows foreshocks and aftershocks associated with the
Landers earthquake (M7.3, 1992) in S. California. The map shows
the area near the Landers earthquake epicenter. Some highways
(dark blue) and surface traces of faults (light bluegreen lines) are
shown. The animation shows earthquake epicenters as colored
spots, appearing and disappearing with time.
SCEC
Fault Creep
Some faults (including the San Andreas and Hayward faults)
exhibit slow gradual displacement known as fault creep.
The displaced curb on D Street in downtown Hayward occurs
along a trace of the Hayward fault. Note that the displacement is
right-lateral.
Displaced curbs such
as this can be seen on
the east side of Mission
Blvd along cross
streets.
The creep rate in
Hayward is
approximately 5 mm/
year.
Five creepmeters are maintained by the
USGS to monitor fault creep on the
Hayward fault.
Fault creep is generally not as
catastrophic as earthquakes, however,
structures that straddle a fault such as
buildings and bridges may be sheared
(ripped apart) by these forces.
The old city hall in Hayward
is an example of a building
that has been destroyed
(condemned) by fault creep.
Creep rates in Fremont
average about 7.8 mm/year.
Creep rates elsewhere on
the fault are close to the ~5
mm/year.
USGS
Seismology
Seismology is the study of
earthquakes.
Seismographs are instruments that
detect and record earthquakes.
The principle behind the seismograph
is that inertia tends to keep the
suspended mass motionless while the
recording surface vibrates with the
bedrock. Thus the seismograph
measures the displacement or
movement of the ground as seismic
waves pass through the station.
Typical seismographs consist of
rotating drums with recording paper.
Most modern seismographs now
record data digitally and are available
in near real time on the internet.
Seismology
Seismology is the study of
earthquakes.
Seismographs are instruments that
detect and record earthquakes.
The principle behind the seismograph
is that inertia tends to keep the
suspended mass motionless while the
recording surface vibrates with the
bedrock. Thus the seismograph
measures the displacement or
movement of the ground as seismic
waves pass through the station.
Typical seismographs consist of
rotating drums with recording paper.
Most modern seismographs now
record data digitally and are available
in near real time on the internet.
Hundreds of seismographs are
deployed in national and
international networks to record
earthquakes.
This extensive network
permits us to determine the
location of an earthquake
and also to accurately
measure the amount of
energy released
(magnitude).
Seismic Waves
How does seismic energy propagate through the Earth?
There are 2 types of seismic waves:
•Surface waves move along the surface of the
Earth. They tend to be the most destructive.
•Body waves travel through the Earth’s interior
and provide useful information about the
earthquake and the interior structure of the
Earth.
There are two types of body waves:
P-waves - primary waves - compress
and extend material in the direction of
wave travel.
S-waves - secondary waves move the
material in a direction that is normal to
the direction of wave travel.
IRIS
P-waves travel ~6 km/sec. They
are compressional waves and
particle motion is in the travel
direction.
Wikipedia: Christophe Dang Ngoc Chan
S-waves travel in the crust ~3.6
km/sec (slower than p-waves).
They propagate through the Earth
by displacing particles
perpendicular to the direction of
Wikipedia: Christophe Dang Ngoc Chan
travel.
P-waves travel ~1.7x faster than S-waves and arrive at a
recording station first. The time delay between the arrival of the
P- and S-waves can be used to determine the distance to the
earthquake.
P-waves travel ~6 km/sec. They
are compressional waves and
particle motion is in the travel
direction.
Wikipedia: Christophe Dang Ngoc Chan
S-waves travel in the crust ~3.6
km/sec (slower than p-waves).
They propagate through the Earth
by displacing particles
perpendicular to the direction of
Wikipedia: Christophe Dang Ngoc Chan
travel.
P-waves travel ~1.7x faster than S-waves and arrive at a
recording station first. The time delay between the arrival of the
P- and S-waves can be used to determine the distance to the
earthquake.
P-waves travel ~6 km/sec. They
are compressional waves and
particle motion is in the travel
direction.
Wikipedia: Christophe Dang Ngoc Chan
S-waves travel in the crust ~3.6
km/sec (slower than p-waves).
They propagate through the Earth
by displacing particles
perpendicular to the direction of
Wikipedia: Christophe Dang Ngoc Chan
travel.
P-waves travel ~1.7x faster than S-waves and arrive at a
recording station first. The time delay between the arrival of the
P- and S-waves can be used to determine the distance to the
earthquake.
Seismograms are records obtained from seismographs. They
provide a lot of information about the earthquake and the portion
of the Earth that the seismic energy has moved through.
Note that this example, the S-wave arrives at this seismograph
station ~500 seconds after the arrival of the P-wave.
The greater the
difference
between the
arrival of the Pand S-waves (SP
interval), the more
distant the
earthquake from
the recording
station.
SP interval
This travel-time graph is used to
determine the distance to the
epicenter of an earthquake. It
shows the travel times for P and
S waves. In addition, it shows
the time lag between the arrival
of the P and S waves (S-P).
In the previous seismogram, the
difference in arrival time
between the P and S waves
was ~36 seconds. That would
correspond to a distance from
the recording station of ~340
km.
This type of analysis can be
done for a single earthquake
from a large number of seismic
recording stations.
Virtual Earthquake (CSULA)
The distances to the
epicenter of the Loma Prieta
earthquake from three
different stations were
determined from
seismograms from Eureka,
CA, Elko and Las Vegas, NV.
On the map, we may draw
circles around each
seismograph station that
represent the distance to the
epicenter.
The epicenter is at the
intersection of the three
circles - it requires at least
three distant seismic
recording stations to
"triangulate" the location of
the epicenter.
Virtual Earthquake (CSULA)
Earthquakes and Plate Tectonics
This plot shows the epicenters of large earthquakes from 1977-92 with
magnitude >5.5. Most occur along narrow belts that are coincident with
plate boundaries. Note that most earthquakes occur around the edge of
the Pacific Ocean. Included in this zone of earthquakes are numerous
volcano chains.
The earthquakes are
color-coded for
depth:
black = shallow
green = intermediate
red = deep
Lamont-Dougherty
The theory of plate tectonics states that
the crust of the Earth is composed of a
strong rigid layer that is broken into 7
major (and many smaller) plates.
Earthquakes occur along these plate
boundaries where they move relative to
one another.
There are three distinct types of plate boundaries:
1. divergent
2. convergent
3. transform
USGS
The theory of plate tectonics states that
the crust of the Earth is composed of a
strong rigid layer that is broken into 7
major (and many smaller) plates.
Earthquakes occur along these plate
boundaries where they move relative to
one another.
There are three distinct types of plate boundaries:
1. divergent
2. convergent
3. transform
USGS
The theory of plate tectonics states that
the crust of the Earth is composed of a
strong rigid layer that is broken into 7
major (and many smaller) plates.
Earthquakes occur along these plate
boundaries where they move relative to
one another.
There are three distinct types of plate boundaries:
1. divergent
2. convergent
3. transform
USGS
This map
shows the
distribution of
earthquakes in
the Pacific
basin.
Each dot
represents the
epicenter of
individual
earthquakes
and are colorcoded for the
depth of the
focus.
Earthquakes at mid-ocean ridges are relatively shallow.
Note the range of earthquake depths in subduction systems on the
western margin of the basin, west coast of S. America and the Aleutian
Islands
USGS
The left diagram shows that as an
oceanic plate is subducted, there is a
deepening zone where earthquakes occur
along the top of the subducted slab
(Benioff zone).
Pacific plate
The right diagram shows the
depths of earthquakes
associated with the subduction
of the Pacific plate beneath the
Philippine plate.
Philippine plate
USGS
33
This map shows the distribution
of earthquakes in California and
Nevada.
Each dot represents the
epicenter of individual
earthquakes and are colorcoded for the depth of the focus.
Note that earthquakes in this
region occur at relatively
shallow depth in the crust.
The zone of earthquakes along
the coast of California are due
to the San Andreas fault system.
The zone of earthquakes in
eastern California is due to
thrust faulting in the Sierra
Nevada.
USGS
Earthquake Intensity and Magnitude
The severity of an earthquake is expressed in terms of the
intensity and magnitude.
The intensity is based on the observed effects of the
earthquake — it is an assessment of the damage caused by an
earthquake at a specific location. Thus the intensity of an
earthquake depends upon the strength of the earthquake, but
also on the distance from the epicenter — it varies from place
to place with respect to the earthquake's epicenter.
The modified Mercalli intensity scale is composed of 12
increasing levels of intensity that range from imperceptible
shaking to catastrophic destruction. It does not have a
mathematical basis but is arbitrary and based on observed
effects.
The levels of the Mercalli scale are given on the next page.
The following is an abbreviated description of the 12 levels of Modified Mercalli intensity.
I. Not felt except by a very few under especially favorable conditions.
II. Felt only by a few persons at rest, especially on upper floors of buildings.
III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many
people do not recognize it as an earthquake. Standing motor cars may rock slightly.
Vibrations similar to the passing of a truck. Duration estimated.
IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes,
windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking
building. Standing motor cars rocked noticeably.
V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects
overturned. Pendulum clocks may stop.
VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster.
Damage slight.
VII. Damage negligible in buildings of good design and construction; slight to moderate in wellbuilt ordinary structures; considerable damage in poorly built or badly designed
structures; some chimneys broken.
VIII. Damage slight in specially designed structures; considerable damage in ordinary
substantial buildings with partial collapse. Damage great in poorly built structures. Fall of
chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
IX. Damage considerable in specially designed structures; well-designed frame structures
thrown out of plumb. Damage great in substantial buildings, with partial collapse.
Buildings shifted off foundations.
X. Some well-built wooden structures destroyed; most masonry and frame structures
destroyed with foundations. Rails bent.
XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.
This map is an
Mercalli intensity
map based on
data collected
from the
community
around
Northridge after
the 1994
Northridge
earthquake
(M6.7).
USGS
37
Earthquake Magnitude
The magnitude of an
earthquake is related to the
amount of energy released
during a seismic event.
The Richter scale is used to
describe earthquake
magnitude. Earthquakes
with magnitudes less than
~2.0 are not commonly felt
by people.
Although there is no upper limit to the Richter scale, the largest
earthquakes have magnitudes of ~9. The energy released by an
earthquake of this size is equal to the detonation of 1 billion tons of
TNT.
The Richter scale is not used to express damage like the Mercalli
scale.
Virtual Earthquake (CSULA)
The Richter
magnitude is
determined from
the maximum
amplitude of
displacement
measured on
seismogram at a
known distance
from the epicenter.
The Richter scale is logarithmic — an increase of 1 on the Richter
scale corresponds to a ~ten-fold increase in the maximum
amplitude (ground motion).
More importantly, each unit on the Richter scale is approximately
equal to a 32-fold increase in released energy. Thus a M7.0
earthquake releases ~32 times more energy than a M6.0.
Earthquake Hazards
Direct hazards
1. ground rupture
2. shaking
Indirect hazards
3. landslides
4. liquefaction and ground
subsidence
5. tsunamis
6. fire
Surface rupture usually occurs
at or near the epicenter of
large earthquakes. Although
damage can be severe from
ground rupture, its areal extent
is usually limited.
The photo shows ground
rupture in the Santa Cruz area
after the Loma Prieta
earthquake.
Ground shaking represents a
much more extensive hazard
since it may affect a much
larger area.
The map indicates the hazard
of ground-shaking where the
colors indicate the amount of
ground-shaking.
J.K. Nakata, , USGS
During a large earthquake,
some areas may receive
more damage than others
due to many different
geologic factors.
As an example, during the
Loma Prieta earthquake,
the Marina district in San
Francisco was more heavily
damaged than other
districts because the area is
underlain by unconsolidated
sediment and artificial fill.
USGS
Liquefaction, is where watersaturated, unconsolidated
sediments behave like a fluid due
to intense shaking of an
earthquake. Liquefaction results
in buildings settling and
collapsing under their own
weight.
Buildings with foundations in
bedrock are generally more
earthquake ready.
Much of the "flat lands" in the
East Bay are susceptible to
liquefaction.
Earthquakes may
provide the energy
to initiate a
landslide.
This map shows
the landslide
susceptibility in
California.
California Geological Survey
Tsunamis are giant
water waves that
usually result from the
vertical displacement
of the seafloor during
an earthquake.
Tsunamis are usually only produced by
earthquakes along convergent boundaries
since these produce the largest
earthquakes and are located in ocean
basins.
USGS
An earthquake can occur in one area and the tsunami may inundate
another thousand of km away. This map shows the propagation of the
Tōhoku tsunami (2011) through the Pacific Basin.
NOAA
The Tōhoku earthquake (2011) caused a 5-8 m
upthrust on the seafloor ~60 km offshore from the
Tōhoku. This resulted in a major tsunami that
propagated across the Pacific.
Click for animation
http://youtu.be/3fqyOpqnJyw
http://youtu.be/q9bKmp77moM
http://youtu.be/jdMDCLwblkY
EERI
A wave height of
~38.9 meters (128 ft)
was estimated at
Omoe peninsula in
Japan.
The coast of Chile
was about 17,000 km
away and was struck
by tsunami waves 2
m (6.6 ft) high.
San Francisco Bay Area
Faults
The San Francisco Bay
Area is tectonically active
and has a large number of
active faults.
These faults are part of the
San Andreas fault system
and all accommodate rightlateral strike-slip motion
along the transform plate
boundary.
A recent study by the USGS
determined that there is a
63% chance of at least one
M6.7 or greater earthquake in
the San Francisco Bay region
between 2007 and 2036.
Note that the Hayward fault
has the highest probability
(31%) of any individual fault
in the region.
This is particularly significant
since the 1989 Loma Prieta
earthquake caused severe
damage in Oakland and San
Francisco even though the
epicenter was more than 50
miles away.
This “shake map” shows
the shaking intensities
for a hypothetical
earthquake on the
southern Hayward fault.
The region of severe to
extreme damage is
widespread and mainly
located around the
margins of the Bay
which is underlain by
unconsolidated
sediments.